The invention relates to a method and a device for operating a rotating machine part which can be actuated by a fluid, for instance the so called front-end chuck of a machine tool. It is known to actuate the opening and/or closing of a chuck—both of which will hereafter be called “operating it”—by pneumatic or hydraulic means, which is particularly advantageous for serially machining workpieces. Because of the required and ever increasing machining speed, it is desirable to be able to command the operation of the chuck not only when it is at rest, but also when its rotation has already begun or not yet stopped.